1. Field of the Invention
The present invention relates generally to image sensors and more particularly, to an image sensor having minimized dark current even with improved operating performance of transistors therein.
2. Background of the Invention
In general, an image sensor is a semiconductor device that generates electrical signals from sensing images. Such image sensors may be categorized as CCD (Charge Coupled Device) image sensors or CMOS (complementary metal oxide semiconductor) image sensors.
A CCD image sensor includes a dense arrangement of MOS capacitors that store and transfer charge carriers. A CMOS image sensor includes a control circuit and a signal-processing circuit surrounding an APS (Active Pixel Sensor) array and is implemented using CMOS technology. The CMOS image sensor includes MOS (metal oxide semiconductor) transistors formed as part of pixels. The outputs from the CMOS image sensor are sequentially output using a switching mechanism.
The APS (Active Pixel Sensor) array of the CMOS image sensor generates electrical signals by detecting light. A logic region surrounding the APS array processes such electrical signals generated by the APS array. Each unit pixel in the APS array includes a respective transfer gate electrode, a respective photodiode disposed to a side of the respective transfer gate electrode, and a respective floating diffusion region.
During operation of an active pixel sensor array, when light reaches a photodiode, electron-hole pairs (EHPs) are generated and accumulated. Such generated charge carriers are transferred to the floating diffusion region via a transfer transistor. As a result, the electric potential at the floating diffusion region changes, and such change of the electric potential is further detected and transferred to the output.
However, the CMOS image sensor may have dark current due to dangling bonds at the surface of the photodiodes. Such dark current deteriorates characteristics of the CMOS image sensor by generating white dots, black dots, and decreasing the operating range.